The ability to see in three dimensions is among the most critical of visual functions. For example, vision is used to guide body movements, which would essentially be impossible without accurate depth perception. However, more than 5% of the US population suffers from stereo-deficiency or abnormal depth perception.
Products to alleviate stereo deficiency on the market include a set of eye-training exercises primarily designed for children. These exercises are collectively called orthoptics. Recent work has emphasized dichoptic training, in which the stimuli to the two eyes have different strengths—i.e., higher contrast in the weak eye and lower contrast in the strong eye. This approach can equalize the input from the two eyes to the brain, and may reduce suppression. Suppression of an eye is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency and aniseikonia. For example, the brain can eliminate double vision by ignoring all or part of the image of one of the eyes. However, merely reducing suppression may not correlate to improved visual function.